1. Field of the Invention
The present invention relates to LED driver, and more specifically, the present invention relates to the control circuit and control method for LED driver with high power factor.
2. Description of Related Art
The offline LED driver normally will use flyback power conversion with primary side regulation for the output current regulation. FIG. 1 shows a prior art of an offline LED driver that has an input electrolytic capacitor 40 for the energy store. As shown in FIG. 1, the conventional offline LED driver includes a rectifier 12. The rectifier 12 receives an input line voltage VAC and rectifies the input line voltage VAC. The input electrolytic capacitor 40 is coupled to an output terminal of the rectifier 12 for the energy store. A voltage VDC is provided by the input electrolytic capacitor 40. A transformer 10 has a primary winding NP, a secondary winding NS and an auxiliary winding NA.
A terminal of the primary winding NP is coupled to receive the voltage VDC. Another terminal of the primary winding NP is coupled to a transistor 20. The transistor 20 is utilized to switch the transformer 10. A terminal of the secondary winding NS is coupled to a terminal of a rectifier 60. An output capacitor 65 is connected between another terminal of the rectifier 60 and another terminal of the secondary winding NS for providing an output voltage VO to a plurality of LEDs 70˜79. The LEDs 70˜79 are connected each other in series and connected to the output capacitor 65 in parallel. A terminal of the auxiliary winding NA is coupled to an anode terminal of a diode 41. A capacitor 45 is coupled between a cathode terminal of the diode 41 and the ground. The auxiliary winding NA charge the capacitor 45 through the diode 41 to generate a power source VCC for a switching controller 50.
The terminal of the auxiliary winding NA is further coupled to a voltage divider. The voltage divider has resistors 51 and 52. The resistors 51 and 52 are connected each other in series. The voltage divider generates a voltage-sense signal VS. The resistor 52 is further coupled to the ground. The switching controller 50 is coupled to a joint point of the resistors 51 and 52 for receiving the voltage-sense signal VS. The switching controller 50 generates a switching signal SW. The switching signal SW controls the transistor 20 to switch the transformer 10 for regulating an output (output current IO and/or the output voltage VO) of the LED driver. When the transistor 20 is turned on, a switching current IP will flow through the transformer 10. Through a resistor 30 coupled to the transistor 20, the switching current IP is utilized to generates a current-sense signal VCS. The current-sense signal VCS is coupled to the switching controller 50.
The waveforms of the input line voltage VAC and the voltage VDC are shown in FIG. 2. The voltage VDC is the voltage on the input electrolytic capacitor 40. The minimum voltage of the voltage VDC will maintain the power conversion operated properly. However, the input electrolytic capacitor 40 causes the distortion of an input current IDC and generate poor power factor (PF). Therefore, the capacitance of the input electrolytic capacitor 40 must be reduced to improve the power factor. However, without the input electrolytic capacitor 40 will cause the voltage VDC to be low. The low voltage of the voltage VDC may cause the feedback open loop for the LED driver. The output voltage VO of the LED driver can be expressed as,
                              V          O                =                  N          ×                      V            DC                    ×                                    T              ON                                      T              -                              T                ON                                                                        (        1        )            where the N is turn ratio of the transformer 10 (N=NS/NP; NP is the primary winding, NS is the secondary winding); the VDC is the input voltage of the transformer 10; TON is the on-time of the transistor 20; T is the switching period of the transistor 20.
In order to achieve a stable feedback loop and prevent the transformer saturation, the maximum duty cycle “TON/T” is limited, such as <80% in general. If the voltage VDC is too low, the maximum on-time TON of the switching signal SW will unable to maintain the output voltage VO (shown in equation (1)) and cause the feedback open loop. When the feedback loop is significantly on/off (close-loop and open-loop) in response to the change of the input line voltage VAC, an overshoot and/or undershoot signal can be easily generated at the output of the LED driver. Besides, the input electrolytic capacitor 40 is an electrolytic capacitor that is bulky and low reliability. The object of this invention is to improve the power factor of the LED driver. Another object of this invention includes eliminating the need of the input electrolytic capacitor 40 for improving the reliability of the LED driver and reducing the size and the cost of the LED driver.